Biology Reference
In-Depth Information
al
. (2001). A
coxAII
disruptant mutant that is defi cient in subunit I of cytochrome
c
oxidase specifi c
to the heterocysts and proheterocysts grew normally under dizotrophic growth conditions with the
differentiation of normal functional heterocysts. This amply signifi es that the loss of one
cox
gene is
compensated by the presence of the other terminal oxidases present in the heterocysts. This is possible
because
Anabaena
sp. strain PCC 7120 appears to possess four types of respiratory oxidase operons,
i.e.
coxBACII
,
coxBACI
,
cyDAB
(a two-subunit operon encoding cytochrome D ubquinol oxidase,
corresponding to genes
all4023
and
all4024
, respectively) and
coxBACIII
. The last two operons were
assigned on the genome of
Anabaena
sp. strain PCC 7120 by Kaneko
et al
(2001).
The expression of
coxBACIII
operon in the developing heterocysts around 9 h after nitrogen
step-down has been reported by Valladares
et al
. (2003). Inactivation of both
coxBII
and
coxAIII
greatly affected nitrogenase activity as well as diazotrophic growth under aerobic conditions but
inactivation of either of these did not matter. This emphasizes the contribution of
coxBACII
and
coxBACIII
complexes to enhanced respiratory activity of the heterocysts to scavenge any oxygen
that sneaks into these cells and the environment conducive for effi cient functioning of nitrogenase is
thus created in these cells (Valladares
et al
., 2003). Heterocyst development in the double
cox
mutant
(
coxBIIcoxAIII
; strain CSAV141) of
Anabaena
sp. strain PCC 7120 was greatly affected at the level
of reorganization of thylakoid membranes and the formation of honey-comb like structure. The
developing heterocysts in CSAV141 also accumulated considerable number of glycogen granules.
Although the deposition of Hgls and the heterocyst envelope polysaccharides is normal and
expression of heterocyst-specifi c
nifHDK
and
fdxH
genes took place, the nitrogenase activity under
aerobic conditions was greatly repressed (Valladares
et al
., 2007). Besides
coxI
that is expressed in
vegetative cells of
A
.
variabilis
ATCC 29413, the expression of
coxII
was up-regulated after nitrogen
step-down in this organism. Single (
coxI
or
coxII
) or double (
coxI
and
coxII
) mutants of
A
.
variabilis
ATCC 29413 exhibited normal diazotrophic growth (Pils
et al
., 2004). The expression of rubrerythrin
(RbrA, a gene product of
alr1174
) in the heterocysts of
Anabaena
sp. strain PCC 7120 that acts like
an FNR-dependent peroxidase confers additional protection against O
2
damage to nitrogenase. A
mutant of
rbrA
differentiated heterocysts but its ability to fi x nitrogen was reduced to 8% to that of the
fi xation rate noted under anaerobic conditions. The heterocysts of the mutant revealed the presence
of high internal concentrations of H
2
O
2
. Further, the expression of
rbrA
from two promoters and one
of the promoters producing the smallest transcript being under the control of HetR favour its role
in the early phases of nitrogenase induction in differentiating heterocysts. These observations are
confi rmed by the presence of very low concentrations of H
2
O
2
in the heterocysts of a double mutant
(
nifH
-
rbrA
-
) than in the vegetative cells (Zhao
et al
., 2007).
In addition, the ability of isolated heterocysts to carry out oxidative phosphorylation (Tel-Or and
Stewart, 1976; Peterson and Wolk, 1978), the dependence of nitrogen fi xation in dark by respiratory
O
2
-uptake (Fay, 1976) and enhanced levels of dehydrogenases (Winkenbach and Wolk, 1973; Apte
et al.
, 1978) constitute evidences for high respiratory metabolism of heterocysts.
6) CYANOPHYCIN
Cyanophycin granule polypeptide [(CGP); multi-L-arginyl poly-(L-aspartic acid)] is a water insoluble
polymer of aspartic acid and arginine (with a molecular weight of 25-100 kD) and serves as a
nitrogen reserve of cyanobacteria (Simon, 1971; Allen, 1984). Lang
et al
. (1972) have demonstrated
that the “structured granules” of
Anabaena
are composed of CGP. In
A
.
cylindrica
the CGP content
of cells increased during stationary phase coinciding with a decrease in soluble protein content and
the addition of chloramphenicol has considerably enhanced the CGP content (Simon, 1973a). CGP
